* {
    padding: 0;
    margin: 0;
}

.box {
    position: absolute;
    width: 100%;
    height: 100%;
    display: flex;
    flex-direction: column;

    header {
        height: 50px;
        background-color: hotpink;
    }

    main {
        flex: 1;
        background-color: orange;
        display: flex;

        nav {
            width: 120px;
            background-color: aqua;
        }

        section {
            flex: 1;
            background-color: chartreuse;
        }
    }
}